1. Field of the Invention
The present invention relates generally to methods of drilling subterranean formations using rotary-type drag bits, and more particularly to such methods employing an oscillating drill bit for more effective removal of formation chips from around the drill bit using drilling fluid.
2. Description of Related Art
Fixed-cutter rotary drag bits have been employed in subterranean drilling for many decades with various sizes, shapes and patterns of natural and synthetic diamonds used on drag bit crowns as cutting elements. Rotary drag-type drill bits typically comprise a bit body having a shank for connection to a drill string and an inner channel for supplying drilling fluid to the face of the bit through nozzles or other apertures. Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal (typically tungsten carbide (WC)) infiltrated at high temperatures with a liquified binder material (typically copper-based) to form a matrix. Such bits may also be formed with layered-manufacturing technology, as disclosed in U.S. Pat. No. 5,433,280, which is assigned to the assignee of the present invention and incorporated herein by reference.
The bit body typically carries a plurality of cutting elements which is mounted directly on the face of the bit body or on carrier elements. The cutting elements are positioned adjacent fluid courses which allow cuttings (i.e., formation chips) generated during drilling to flow from the cutting elements to and through junk slots on the gage of the bit. The cuttings then move to the borehole annulus above the bit. Cutting elements may be secured to the bit by preliminary bonding to a carrier element, such as a stud, post, or cylinder, which is, in turn, inserted into a pocket, socket, recess or other aperture in the face of the bit and mechanically or metallurgically secured thereto.
One type of drag bit includes polycrystalline diamond compact (PDC) cutters typically comprised of a diamond table (usually of circular, semi-circular or tombstone shape) which presents a generally planar cutting face. A cutting edge (sometimes chamfered or beveled) is formed on one side of the cutting face which, during boring, is at least partially embedded into the formation so that the formation impacts at least a portion of the cutting face. As the bit rotates, the cutting face contacts the formation and a chip of formation material shears off and rides up the surface of the cutting face. When the bit is functioning properly, the chip breaks off from the formation and is transported out of the borehole via circulating drilling fluid. Another chip then begins to form in the vicinity of the cutting edge, slides up the cutting face of the cutting element, and breaks off in a similar fashion. Such action occurring at each cutting element on the bit removes formation material over the entire gage of the bit, and thereby causes the borehole to become progressively deeper.
In some subterranean formations, PDC cutting elements are very effective in cutting the formation as the drag bit rotates and the cutting edge of the cutting element engages the formation. However, in certain formations exhibiting plastic behavior, such as highly pressurized deep shales, mudstones, siltstones, some limestones and other ductile formations, the formation chips have a marked tendency to adhere to the leading surface of the bit body and the cutting face of the cutting element.
When formation chips adhere to the cutting elements, fluid courses or junk slots of the drill bit, the accumulated mass of chips impedes the flow of drilling fluid to the cutters and impedes the flow through the fluid courses and junk slots resulting in the reduction of cooling efficiency of the drilling fluid. Additionally, adherence of formation chips at or near the cutting faces of the cutting elements can actually prevent chips from sliding over the cutting face resulting in reduced cutting efficiency.
When these formation chips adhere to the cutting face of a cutting element, they tend to collect and build up as a mass of cuttings ahead of and adjacent to the point or line of engagement between the cutting face of the PDC cutting element and the formation, potentially increasing the net effective stress of the formation being cut. The buildup of formation chips moves the cutting action away from and ahead of the edge of the PDC cutting element and alters the failure mechanism and location of the cutting phenomenon so that cutting of the formation is actually effected by the built-up mass, which obviously is quite dull. Thus, the efficiency of the cutting elements, and hence of the drag bit itself, is drastically reduced.
Undesired adhesion of formation cuttings to the PDC cutting elements has long been recognized as a problem in the subterranean drilling art. A number of different approaches have been attempted to facilitate removal of formation cuttings from the cutting face of PDC cutting elements. For example, U.S. Pat. No. 5,582,258 to Tibbitts et al., assigned to the assignee of the present invention and herein incorporated by this reference, includes a chip breaker formed adjacent the cutting edge of the cutting elements to impart strain to a formation chip by bending and/or twisting the chip and thereby increasing the likelihood that the chip will break away from the face of the bit. Other approaches to solving the problem of formation chip removal include U.S. Pat. No. 4,606,418 to Thompson which discloses cutting elements having an aperture in the center thereof which feeds drilling fluid from the interior of the drill bit onto the cutting face to cool the diamond table and to remove formation cuttings.
U.S. Pat. No. 4,852,671 to Southland discloses a diamond cutting element which hag a passage extending from the support structure of the cutting element to the extreme outermost portion of the cutting element, which is notched in the area in which it engages the formation being cut so that drilling fluid from a plenum on the interior of the bit can be fed through the support structure and to the edge of the cutting element immediately adjacent the formation. U.S. Pat. No. 4,984,642 to Renard et al. discloses a cutting element having a ridged or grooved cutting face on the diamond table to promote the break-up of formation chips, or in the case of a machine tool, the break-up of chips of material being machined, and enhance their removal from the cutting face. The irregular topography of the cutting face assists in preventing balling or clogging of the drag bit by reducing the effective surface or contact area of the cutting face, which also reduces the pressure differential of the formation chips being cut. U.S. Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of the present application, employs ridged, grooved, stair-step, scalloped, waved and other alternative non-planar cutting surface topographies to permit and promote the access of fluid in the borehole to the area on the cutting element cutting face immediately adjacent to and above the point of engagement with the formation. Such a non-planar cutting surface helps to equalize differential pressure across the formation chip being cut and thus reduce the shear force which opposes chip movement across the cutting surface.
U.S. Pat. No. 4,883,132 to Tibbitts, assigned to the assignee of the present application, discloses a novel drill bit design providing large cavities between the face of the bit and the cutting elements engaging the formation. Formation cuttings entering the cavity area are thus unsupported and more likely to break off for transport up the borehole. In addition, clearing of the cut chips is facilitated by nozzles aimed from behind the cutting elements (taken in the direction of bit rotation) so that the chips are impacted in a forward direction to break off immediately after being cut from the formation. U.S. Pat. No. 4,913,244 to Trujillo, assigned to the assignee of the present invention, discloses bits which employ large cutters having associated therewith directed jets of drilling fluid emanating from specifically oriented nozzles placed in the face of the bit in front of the cutting elements. The jet of drilling fluid is oriented so that the jet impacts between the cutting face of the cutting element and a formation chip as it is moving along the cutting face to peel the chip away from the cutting element and toward the gage of the bit. Likewise, GB 2,085,945 to Jurgens provides nozzles that direct drilling fluid toward the cutting elements to flush away cuttings generated by the cutting elements.
U.S. Pat. No. 5,447,208 to Lund et al., assigned to the assignee of the present invention, discloses a superhard cutting element having a polished, low friction, substantially planar cutting face to reduce chip adhesion across the cutting face. U.S. Pat. No. 5,115,873 to Pastusek, assigned to the assignee of the present application, discloses yet another manner in which formation cuttings can be removed from a cutting element by use of a structure adjacent to and/or incorporated with the face of the cutting element to direct drilling fluid to the face of the cutting element and behind the formation chip as it comes off the formation.
It has also been disclosed in the art that drilling systems which employ cycloidal sonic energy as a method of drilling cause highly effective cutting action on the bottom and particularly the adjacent side walls of the bottom portion of the well bore by virtue of the cycloidal drilling action. Typically, such vibratory drilling systems employ orbiting mass oscillators to generate vibratory energy. Such orbiting mass oscillators may employ orbiting rollers which are rotatably driven around the inner race wall of a housing, as disclosed in U.S. Pat. No. 4,815,328 to Bodine, or an unbalanced rotor, the output of which is coupled to a drill bit, as disclosed in U.S. Pat. No. 4,261,425 to Bodine. U.S. Pat. No. 5,562,169 to Barrow discloses a sonically driven drill bit employing an oscillator adapted to transmit sinusoidal pressure waves through the drill pipe.
None of the foregoing approaches to cutting element and bit design have been completely successful in facilitating chip removal from the face of the cutting element. Moreover, it will be appreciated by those skilled in the art that many of the foregoing approaches require significant modification to the cutting elements themselves, to the structure carrying the cutting elements on the bit face, and/or to the bit itself. Thus, many of the foregoing approaches to the problem require significant expenditures which substantially raise the price of the drill bit. In addition, due to required cutter placement on certain styles and sizes of bits, many of the prior art hydraulic chip removal arrangements are unsuitable for general application. Moreover, those bits employing vibrating drilling systems do not address the problem of chip removal. Accordingly, it would be extremely desirable to provide the industry with a solution to the impairment to the cutting mechanism caused by chip adhesion, which solution could be economically employed in any drill bit regardless of size or style, and regardless of the type of formation which might be encountered by the drill bit.
In accordance with the present invention, drilling apparatus is provided for effecting a drilling method in which formation chips are produced with varying thicknesses to promote fracturing of the formation chips, thereby avoiding the buildup of formation chips near the bit body and facilitating removal of the formation chips from the bit face. Formation chips having various thicknesses are produced by selectively modifying the degree to which the cutting elements of the bit contact and cut the formation. Selective modification of the degree to which cutting elements contact the formation is achieved in the present invention by essentially modifying the axial and/or rotational/torsional movement of the drill bit, portions of the drill bit or the cutting elements attached to the drill bit.
The present invention provides apparatus for drilling a subterranean formation employing, by way of example only, a rotary-type drag bit comprising a bit body having a plurality of longitudinally extending blades, where adjacent blades define fluid courses with communicating junk slots therebetween. A plurality of cutting elements is attached to the blades, each cutting element including a cutting face oriented toward a fluid course. Upon rotation of the drill bit in a subterranean formation, formation chips cut by the cutting elements slide across the cutting elements, into the fluid courses and through the junk slots. The formation chips are then flushed into the annulus of the borehole.
In accordance with the drilling methods of the present invention, movement of the drill string, bit body or cutting elements is modified in a manner which introduces weak points into the formation chips as they are cut from the formation. That is, varying thicknesses are introduced into each formation chip as it is cut, thereby facilitating preferential breaking of the chip. In one embodiment, the bit is structured to oscillate torsionally as it rotates to produce alternating, relatively thicker and thinner sections of the chip such that each thicker chip portion is more likely to break away from the rest of the chip along the thinner portions of the chip by the force of drilling fluid contacting the chip. The broken formation chips enable their removal from the bit body and the borehole. Oscillation can be achieved by, for example, vibrating a near-bit sub or the bit shank using, for example, unbalanced rotating masses or an oscillating motor having an unbalanced rotor. In addition, such torsional oscillations may be produced at the surface by using a slip clutch in a near-bit sub, at the top drive, or in association with the rotating table. A pulsing hole wall brake, which cyclically engages and disengages the wall of the well bore, or a near-bit sub having a rotational transmission device which cyclically engages and disengages the drill bit may also oscillate the rotational velocity of the rotating drill bit. In harder formations, a cavitation jet which creates an irregular turbulent flow of drilling fluid around the bit, the flow direction of which oscillates, may cause vibration and, thus, may cause rotational oscillation of the bit relative to the well bore. Finally, a drill bit having individually oscillating cutting elements induced by increasing and decreasing drilling fluid pressure to the cutting elements may be employed to achieve the desired torsional oscillation.
In another embodiment of the invention, the bit is vertically oscillated relative to the longitudinal axis of the bit such that the load on the drill bit is cyclically increased and decreased to effect alternating deeper and relatively more shallow cuts into the formation, thus varying the thickness of formation chips generated by the cutting elements. Such vertical oscillations may be affected by varying the weight on bit (WOB) at the top drive. In addition, vertical oscillations may be accomplished by employing a fluid pulse to cyclically create alternating higher and lower hydrostatic pressures in the bit to cause variable degrees of contact with the formation. This may be accomplished by employing a valve and fluid jet assembly on a near-bit sub to xe2x80x9cpulsexe2x80x9d the drill bit vertically or at an angle, or by employing a valve and a piston-like assembly in or above the drill bit to cyclically vary the depth of cut (DOC) of the drill bit into the formation. In addition, a drill bit which is resiliently attached to the drill string, such as by a spring-loaded bit sub or piston-like bit sub which can vertically oscillate the bit relative to its longitudinal axis, can cyclically vary the depth of cut of the bit into the bottom of the borehole to produce formation cuttings of different thicknesses. Vertical oscillation in the cutting elements may also be produced by structuring a bit having adjustable blades.
In yet another embodiment of the invention, both vertical and torsional oscillation may be imposed on the drill bit by combining devices that produce vertical oscillation with those that produce torsional oscillation. Likewise, drill bit oscillation that is neither completely torsional nor completely vertical, but at some angle to the longitudinal axis of the drill bit, may be produced by combining devices herein describe or by operating a single device, such as a fluid pulse, at an angle to the longitudinal axis of the drill bit.